The present invention relates to a portable terminal such as a cellular phone or PHS which is equipped with the GPS function of specifying the current position of the terminal by receiving radio waves from a GPS (Global Positioning System) satellite and, more particularly, to a GPS time keeping method of keeping a GPS time.
Recently, portable terminals such as cellular phones and PHSs which have self position search functions based on the GPS have been proposed. FIG. 11 shows a conventional portable terminal 3 equipped with such a GPS function.
As shown in FIG. 11, the conventional portable terminal 3 includes a communication oscillator 10, GPS oscillator 20, communication processor (C-CPU) 40, and GPS processing unit 150.
The communication oscillator 10 is an oscillator for generating clocks to the communication processor 40. The GPS oscillator 20 is an oscillator for generating clocks to the GPS processing unit 150. The clocks generated by the communication oscillator 10 are input as operation clocks to the communication processor 40. The clocks generated by the GPS oscillator 20 are input as operation clocks to the GPS processing unit 150.
The GPS processing unit 150 receives a radio wave from each GPS satellite, and calculates the distance to each GPS satellite from the difference between the time information contained in this radio wave and the time when the radio wave was received. In order to perform high-accuracy position measurement, both the time in a GPS satellite on the transmitting side and the time in a GPS receiver on the receiving side must be accurate.
Each GPS satellite is equipped with an atomic clock, and hence the time information contained in the radio wave transmitted from each GPS satellite is high in accuracy. It is, however, difficult to mount such a high-accuracy timepiece in a GPS receiver. It is physically impossible for a device required to achieve reductions in size and power consumption, e.g., a portable terminal, in particular, to have a timepiece means with as high accuracy as an atomic clock.
For this reason, a general GPS receiver obtains a high-accuracy time by using the time information contained in radio waves received from a plurality of GPS satellites. The distance to each GPS satellite is calculated using this high-accuracy time. The time calculated from the radio wave from each GPS satellite is called the GPS time.
Since this GPS time is very accurate, various types of apparatuses designed to perform high-accuracy clocking by using the GPS time have been proposed (see, for example, Japanese Patent Laid-Open No. 2002-148372). A radio communication system has also been proposed, in which both a transmitter and a receiver use the GPS time to establish synchronization so as to be synchronized with each other.
However, not all general portable terminals such as cellular phones and PHSs (Personal Handy-phone Systems) are equipped with the GPS function. For this reason, a communication scheme asynchronous with the GPS time is used between a base station and a cellular phone. Assume that the GPS function is provided in a cellular phone based on a communication scheme asynchronous with the GPS time. In this case, if the GPS time is calculated for each position measurement, it takes much time for position measurement. For this reason, the GPS time is kept once it is obtained, and next position measurement is performed by using the kept GPS time, thereby shortening the position measurement time and improving the position measurement performance. A portable terminal is, however, required to save power consumption, and hence may be shifted to the power saving mode. In addition, the power of the cellular phone is not always ON, and is turned off in some cases. Note that in this specification, all the cases wherein the power is shut down, e.g., a normal power saving mode and power-off state, will be expressed as power saving modes.
It is generally known that knowing a more accurate GPS time makes it possible to acquire the GPS time more quickly after the start of position measurement processing, and acquiring the GPS time more quickly makes it possible to improve the position measurement performance. For this reason, even if the power saving mode is set, keeping a time as close to the GPS time as possible can shorten the position measurement time after returning from the power saving mode.
A conventional method of keeping the GPS time when such a power saving mode is activated will be described with reference to FIG. 12.
FIG. 12 shows, on the GPS time axis, processing from the time the GPS processing unit 150 finishes position measurement (time T1) to the time the portable terminal shifts to the power saving mode (time T2), returns from the power saving mode (time T3), and then starts the next position measurement (time T4). FIG. 12 shows two examples.
The <first example> will be described first. At time T1, position measurement is finished. At this time, time T1 is also set as the GPS time in the GPS processing unit 150. However, the GPS processing unit 150 does not keep the GPS time when position measurement is finished. At the time point when the next position measurement is started, the GPS time remains as time T1 in the GPS processing unit 150.
The <second example> will be described next. At time T1, position measurement is finished. At this time, time T1 is also set as the GPS time in the GPS processing unit 150. Thereafter, the GPS processing unit 150 continuously keeps the GPS time by using a GPS oscillator frequency from the GPS oscillator 20 until the portable terminal enters the power saving mode or the power is turned off.
When the GPS processing unit 150 enters the power saving mode, the unit stops keeping the GPS time. After the portable terminal returns from the power saving mode, the GPS time in the GPS processing unit 150 remains as time T2. After the portable terminal returns from the power saving mode, the GPS processing unit 150 continuously keeps the GPS time by using the GPS oscillator frequency. At the time of the start of the next position measurement, however, the GPS time in the GPS processing unit 150 becomes T2+(T4−T3).
According to the conventional method of keeping the GPS, however, in the second example as well, the GPS time cannot be kept during the power saving mode, and hence an accurate GPS time cannot be known at the time of the start of the next position measurement.
A method of keeping the GPS time with high accuracy even if no GPS time information can be received from a GPS satellite is disclosed (see, for example, Japanese Patent Laid-Open No. 2000-332678). This method is, however, designed to adjust the oscillation frequency of an oscillator in an apparatus during reception of GPS radio waves, and hence requires a circuit for adjusting the oscillation frequency. In addition, this method is required to make the oscillator continuously operate, and hence cannot be applied to the above system designed to shift to the power saving mode.
The above portable terminal equipped with the conventional GPS function cannot keep the GPS time during the power saving mode. It takes therefore much time to perform position measurement after returning from the power saving mode.